Adjustable resistance device



May 25, 1965 L. E. wEYENBl-:RG 3,185,950

ADJUSTABLE RESISTANCE DEVICE Filed Oct. 23. 1961 IN VEN TOR.

Lionel E. Weyenberg /ndr'us Sfar/ye Mge@ United States Patent O3,155,950 ADJUSTABLE RESSTANCE DEVICE Lionel E. Weyenherg, 3845 N. 169thSt., Brookfield, Wis. Filed (let. 23, 1961, Ser. No. 146,964 8 Claims.(Cl. SSS-JVM) This invention relates to an Iadjustable resistancedevice, particularly power rheostats, high temperature rheostats and thelike, and to the method of making the same.

In electrical circuits, variable resistance devices such las a powerrheostat may be connected in the circuit tor varying the voltage,cur-rent, or other characteristic thereof. A power rheostat generallyincludes a resistance coil wound on .a suitable core or support. One endof the coil is connected in a circuit and :a -contact `or wiper isslidab'ly mounted on the coil `and connected in the circuit to provide'a means of adju-sting the portion of the lrheostat coil which isinserted into the circuit. Power rheostats lare normally designed tocarry substantial current and must be oi a high temperature variety.Current in a conductor `generates heat in :accordance with the lsquareof the current and the resistance of the wire and as a result,substantial amounts of heat are generated within a rheostat coil. Theamount of current or power that the rheostat can carry is directlydependent upon the dissipation of the heat generated within therheo-st-at. Consequently, power rheostats are normally not enclosed inprotective housings but rather tare mount-ed for free circulation of theair or the like to establish rapid dissipation of the heat. Powerrheostats 4are consequently `subject to the deteriorating eti-ect of theforeign elements in the :surrounding atmosphere.

As the proportion of the resistance coil connected in the circuitdecreases, the current through the coil normally increases. ln order tocarry normal full load current, power rheostats have heretofore requiredthat a substantial portion of the rheostat be connected in the circuit.

Totally enclosed rheostats and the like have been provided ter verysmall ratings wherein heat dissipation is not a severe restriction. Forexample, Unite-d States Patent No. 2,870,304 to OBrian discloses ahelically wound resistor coil mounted within an insulating protectivehousing. A ring contact is secured to a threaded follower in sliding`engagement with the coil. In response to rotation of the shaft, thering -contact is selectively positioned yupon the resistance coil.Rheostats as shown in 'OBrian, are satisfactory for relatively smellwattages; for

example, in the range of 5 watts or the like.

In power rheostats which normally vare rated in terms of 100 watts ormore, totally enclosed rheostats have not been satisfactory because ofthe resulting heat concentrations.

T he present invention provides `an 'adjustable resistance device havingan enclosing hou-sing and adapted for high power ratings. Thepresentinvention further provides `an improved method of forming at-otally enclosed power rheostat as well ras other rheostats and similardevices in an inexpensive and rapid manner.

In accordance with the present invention a high heat dissipating housingof suitable metal or the like is provided. A resistance coil is `bonded.to the inner wall of the housing by an electrically insulating and hightemperature compound which `also has good heat transfer characteristics.The compound .intimately bonds the coil 3,185,950 Patented May 25, 1965ICC to the housing and creates a rapid transfer of the heat to thehousing. The generated heat is rapidly distributed throughout thehousing `and then dissipated -to the surrounding atmosphere. Applicanthas found that the coil can be totally enclosed within a suitable-dustp-roof enc'losurefwithout .requiring abnormal large coolingsurfaces while maintaining power ratings of wattages of one hundredwatts and the like. T he construction of the present invention permitscarrying full load with .as much as 75 percent of resistance coilremoved from the circuit.

In accordance with another aspect of this invention, a pair of opposedsliding contacts are secured toen axially moving positioning shaft. Theopposed sliding contacts allow adjustment in half turns of the coil forrelatively fine control. If either one of the sliding contacts burnsout, or is otherwise destroyed, the other contact maintains operati-on,until the `defective contact brush can lbe replaced or repaired.

lt is important that the rheostat winding be firmly and intimatelybonded to the heat dissipating and enclosing housing. A particularlyhighly satisfactory and novel method oi securing a winding such as therheostat winding of the present invention in a housing includes thesteps of winding a coil on a mandrel preferably having resilient facingsadjacent the areas with which the sliding contacts engage. A layer ofthe bonding compound is applied over the outer circumference of the coiland the mandrel and allowed to set. The compound actually iows betweenthe adjacent convolutions of the coil and securely embeds the coilwithin the compound, except adjacent the resilient facing whichmaintains the adjacent face of the coil free from the compound. The coilsupported by the outer layer of compound is properly located within thehousing and an additional or second layer of compound is filled betweenthe housing and the back side of the iirst layer. The second layer isallowed to set and securely and intimately bond the coil to the housing.

rl`he present invention provides a lightweight, compact rheostat orother similar electrical device and is particularly adapted for formingof totally enclosed resistance devices having power or high temperatureratings. The device introduces a minimum amount of reactance into thecircuit and can carry full wattage ratings with only small portions ofthe resistance coil or the like connected in the circuit. 'K

The drawing furnished herewith illustrates the best mode presentlycontemplated for carrying out the invention.

In the drawing: j

FIG. l'is a side elevational view of a totally enclosed rheostat;

FIG. 2 is a vertical longitudinal section of the rheostat shown in FIG.l;

FIG. 3 is a fragmentary View taken on lines 3-3 of FG. 2;

FIG. 4 is a radial section taken on lines 4 4 of FIG. 2;

FIG. 5 is a diagrammatic illustration showing the initial winding of arheostat coil such as shown in FIGS. 2 and 3;

FIG. 6 is a diagrammatic illustration showing the application of aninitial layer of compoundupon a wound coil; and

FIG. 7 is a diagrammatic illustration showing the assembly of a woundcoil within a housing and the application of the final bonding compoundto secure the coil within the housing as shown in FIGS. 1 4.

Referring to the drawing and particularly to FGS. 1 4, a power rheostatl of general cylindrical shape is shown having a pair of input terminals2 and a pair of output terminals 3 secured to one end and an input shaftor rod 4 slidably mounted in the opposite end thereof. A resistance coil5 of relatively tine wire is secured within the rheostat l and connectedat its opposite ends to the terminals 2. A pair of opposed brushes 6 and.7 slidably engage coil 5 and are mounted on opposite ends et a tubularbrush holder 8 which is connected to the inner end of input shaft or rod4 for selective positioning of the brushes 6 and 7 on the coil 5 byaxial movement of the rod li. The brushes 6 and 7 are connected to theterminals 3, as hereinafter described, to vary the proportion of thecoil 5 connected between the respective terminals 2 and the brushes 6and 7 for insertion into a suitable electrical circuit, not shown.

Referring particularly to FIGS. 2 and 4, the illustrated rheostat llincludes a cylindrical housing or casing 9 formed of a suitable metalhaving good heat dissipating characteristics. Aluminum, magnesium andother similar lightweight metals have been found particularlysatisfactory. End walls lit and 11 are removably secured to thecorresponding ends of the casing 9 to provide an airtight enclosure forcoil 5. The end walls il@ and il project into the casing 9 and areSecured in place by a plurality of circumferentially distributed setscrews i2 which extend through suitably located openings in the ends ofcasing 9 and thread into suitably tapped openings in the end walls andil.

The resistance coil 5 includes a single layer of helicaily wound closelyspaced convolutions which are bonded to the inner wall of the casing 9by a suitable layer of cement 13. rThe coil 5 is formed of a suitableresistance wire for carrying selected currents with suitable dissipationof heat from the coil.

The cement 13 is any suitable cement having goed electrical insulatingand heat transfer characteristics in order to both insulate the coil 5from the metal casing 9 and to rapidly transfer the heat from the coil 5to the casing 9 from which the heat is dissipated to the suroundingatmosphere. The cement ll3 is spaced inwardly at the ends of the casing9 and at 14 and i5 to allow attachment of the end walls 10 and 11.

The cement 13 extends between the convolutions of the coil 5 and overthe inner face of portions of coil 5 as at 16 to physically lock theindividual convolutions of the coil 5 together and to firmly support thecoil within the casing 9.

The coil 5 is provided with a pair of diametricaliy opposed coil facings17 and 18 which are free of all cement 13 and suiiciently wide toaccommodate the brushes 6 and 7.

Suitable leads 19 and Ztl connect the opposite ends of the coil 5 to theterminals 2 for connection into a circuit.

The terminals 2 are diametrically spaced from each other, as mostclearly shown in FIG. 4, and formed by a threaded screw which passesthrough a suitable opening in the end wall 19. A hex nut 21 is threadedonto each of the screw terminals 2 and locks the corresponding terminalto the end wall it). Suitable insulation 22 is disposed between theterminals 2 and the wall l() to electrically isolate the terminals fromthe casing 9. When an input is connected to either one of the terminals2, the current path is through the connected end portion of the coil 5to and through the brushes 6 and 7 to the terminals which are connectedto brushes 6 and 7, as hereinafter described.

The illustrated brushes 6 and 7 are generally conventional rectangularcarbon blocks which are slidably disposed within the opposite ends ofthe tubular brush holder 8 and slidably engage the diametricallyopposite portions v17 and 18 of the coil 5.

The tubular brush holder 8 is somewhat shorter than the inner diameterof the coil 5 to allow free movement ot the brush holder 8 through thecoil and includes open ends corresponding to the cross-sectionalconfiguration of the brushes 6 and 7. The brush holder 8 includes acentral opening accommodating a reduced threaded end 23 of the rod 4. Ahex nut 24 is threaded onto the rod 4 and locks the holder to the rod d.Insulation 25 is dis posed between the connection of rod d and holder 8to electrically isolate the rod from the holder.

The rod i is slidably journaled in a tubular bearing member which issecured within a suitable opening in the end wall l. Lock nuts 27 and 28are threaded on the hearing member 26 on opposite sides of the end wallill to rigidly lock the bearing member 26 in place. The tubular bearingmember 26 preferably extends axially outwardly out of the end wall 11and serves as a mounting means for the rheostat.

The holder 8 is guided within the casing 9 for rectilinear movement bythe guide rods 29 and 3@ which project axially of the casing 9 and passthrough correspondingly spaced guide openings in holder 8 on oppositesides of the rod 4. Each of the guide rods 29 and 30 is secured as anextension of one of the output terminals 3 on the end wall i0.

The terminals 3 are generally threaded members from the wall l@ similarto terminals 2 and extend through suitable openings in the end wall rilwith a locking hex nut 31 threaded on the outer ends. insulation 32 isdisposed betwcen the end wall lil and the terminals 3 to electricallyisolate the terminals and the guide rods Z9 and Sil from the end walllil and casing 9.

Referring particularly to FIG. 4, the terminals 3 and therefore guiderods 29 and 3@ are located in a plane perpendicular to the planeincluding the terminals 2 and generally bisecting the exposedcement-free tacings of the coil 5. The holder 3 and brushes 6 and 7 areconsequently aligned with the cement-free coil acings 17 and 1S.

The brushes 6 and 7 are continuously biased from the holder 8 intoengagement with the facings t7 and i3 by coil springs 33 and 34 whichare provided within the holder S. Contact blocks 35 and 3d are disposedbetween the inner end of the coil springs 33 and 34 respectively and theadjacent guide rods 29 and 30. In the assembled position, the springs 33and 34 are compressed between the brushes 6 and 7 and the correspondingContact blocks 35 and 36 to resiliently hold brushes 6 and '7 in firmsliding engagement with coil 5 and blocks 35 and 36 in iirm slidingengagement with rods 29 and 3l).

Similar jumper leads 37 and 38 connect the respective contact blocks 35and 36 to the brushes 6 and 7 and establish a low resistance paththerebetween. Current from the brushes 6 and 7 is connected through thejumper leads 39 and 4d and contact blocks 35 and 36 to the guide rods 29and 30 and then to the terminals 3.

The axial positioning or the rod 4 within the casing 9 determines theposition of the brushes 6 and 7 with re spect to the coil 5 andconsequently the portion of the resistance coil 5 connected in thecircuit.

The current through the coil 5 results in generation of heat inproportion to the square of the current and the resistance of the coil5. As previously noted, the rate of dissipation of the heat constitutesone limiting factor on the rating of the rheostat. In accordance withthe illustrated embodiment of the invention, the heat generated withinthe coil 5 is rapidly transferred by the cement t3 to the casing 9 fromwhich the heat is rapidly dissipated to the surrounding atmosphere.Applicant has found that a rheostat constructed in accordance with theabove teachings can carry full load with percent of the windingdisconnected from the circuit. The present invention provides a lightand compact power rated rheostat having an outer dustproof housing.

The brushes 6 and 7 result in a two point contact, individuallyconnected into the circuit through the separate 5 guide rods 29 and 30.If either one of the brushes 6 and 7 malfunctions or is destroyed, theopposite brush maintains the connection into the circuit.

Referring to FIGS. 5-7, a preferred method of forming the illustratedembodiment of the invention of FIGS. 1-4 is illustrated.

Referring particularly to FIG. 5, a winding mandrel 39 is providedhaving an outer diameter substantially corresponding to the innerdiameter lof the coil 5. Resilient strips 40 and 41, corresponding inwidth to the exposed facings 17 and 18 of the coil S in FIGS. l-4, aresecured on diametrically opposite sides of thekmandrel 39. A wire 42 iswound on the mandrel 39 and strips 40 and 4l to form the coil 5. k

After the wire 42 has been completely wound on the winding mandrel 39 toform coil 5, a layer of cement 43 is applied over the back side of thecoil, as shown in FG. 6. The layer of cement 43 may be applied directlyinany suitable manner, with or without a forming container.

Theinner layer of cement 43 moves between and at least partially aroundthe convolutions of coil 5 throughout the circumference of coil 5 exceptat the area of the resilient strips 40 and 41 which establish thecement-free facings 17 and 1S without additional operations. Theconvolutions are tightly locked in place by the cement to provideexcellent support during operation of the rheostat. The cement 43adjacent the facings 17 and 18 bonds to the outer surfaces of theconvolutions of coil 5 to support those portions of coil S.

The cement 43 is allowed to harden and the mandrel 39 may then beremoved. The mandrel 39 may be of break-away, collapsible constructionor other suitable removable variety.

rfhe coil 5 and the hardened cement layer 43 thereon is then properlylocated within a casing 9 to coaxially align the coil 5 within thecasing 9 of greater diameter than the cement 43. A cement 44 isintroduced to fill the space 45 between the back side of the cementlayer 43 and the casing 9. The cement 44 is allowed to harden and in sodoing bonds to the inner wall ofthe casing 9 and to the cement layer 43to form a continuous layer of cement adhesively bonding the coil 5 tothe casing 9.

The end walls 1t) and 1l and other properly located elements areattached to form the totally enclosed rheostat 1. The cement 43 and 44in addition to physically supporting the coil 5 provides rapid andeffective heat transfer of the generated heat to the casing 9 from whichit can be dissipated to the surrounding atmosphere.

The present invention provides a totally enclosed rheostat tor similardevice which is compact and relatively inexpensive which can be readilyproduced employing modern mass production techniques.

Various modes fof carrying out the present invention are contemplated asbeing within the scope of the following claims particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention.

I claim:

l. In an adjustable electrical resistance device, a tubular metalliccasing, a resistance wire coil within the casing, a bonding andinsulating material securing the coil to the casing, said coil includingat least diametrically opposed facings free of said material, a brushholder slidably mounted within the casing and movable axially therein,and a pair of contacts resiliently secured in the brush holder inindividual engagement with the opposed facings.

2. The construction of claim l having separate output connections forsaid brushes.

3. In an adjustable electrical resistance device, a tubular metalliccasing, a resistance wire coil within the casing, an electricallyinsulating bonding cement securing the coil to the inner wall of thecasing, said coil having a cement-free facing extending generallyaxially thereof,

a rod-like brush holder slidably mounted to the casing and movableaxially therein, a guide member disposed within the housing andconnected to the brush holder, means coupled to the brush holder andcooperating with said guide member to prevent rotation of the brushholder, and a brush resiliently secured in the brush holder inengagement with the cement-free facing.

4. In an adjustable electrical resistance device, a tubular metalliccasing, removable metallic end walls to form a substantially closedenclosure, a resistance wire coil of spaced convolutions disposed withinthe casing, a layer of a bonding and insulating cement securing the coilto the inner wall of the casing and extending between and about theconvolutions except in a selected contact path, a brush holder slidablydisposed within the casing and movable axially therein, and a pair ofbrushes resiliently secured in the brush holder in individual engagementwith the opposed cement-free facing.

5. An adjustable rheostat, comprising a tubular metallic casing havingremovable metallic end walls, a coil of resistance Wire within thecasing, a layer of electrically insulating and heat transmitting cementbonding the coil to the casing and supporting the coil, said cementextending between the convolutions of the coil and overlying the face ofthe coil, a pair of diametrically opposed cementfree coil facings, apair of brush terminals secured to the end wall in the plane of thecement-free coil facings and projecting axially inwardly of the casing,a tubular brush holder, an axially movable rod journaled in an end walland secured to the brush holder for selective positioning of the holder,a pair of guide rods secured one each to said brush terminals andprojecting axially to the opposite end of the casing, said brush holderhaving guide rod openings accommodating said guide rods to slidablysupport the brush holder with the ends thereof aligned with said coilfacings and to prevent rotation thereof, contact brushes slidablydisposed within the ends of the brush holder, and resilient meansdisposed within the tubular member and resiliently holding the contactbrushes in engagement with the exposed facings.

6. An adjustable rheostat, comprising a tubular metallic casing havingremovable metallic end walls, a coil of resistance wire within thecasing, a layer of electrically insulating and heat transmitting cementbonding the coil to the casing and supporting the coil, said cementextending between the convolutions of the coil and overlying the face-of the coil, a pair of diametrically opposed cement-free coil facings,a pair of coil terminals secured to one end Wall in a plane normal tothe longitudinal plane through the cement-free coil facings andconnected one each to opposite ends of the coil, a pair of brushterminals secured to the end wall in the plane of the cement-free coilfacings, a tubular brush holder, an axially movable positioning rodjournaled in the end wall opposite said terminals and centrally securedto and in electrically insulated position from the brush holder forselective positioning of the holder, a pair of guide rods secured oneeach to said brush termials and projecting axially to the opposite endof the casing, said brush holder having guide rod openings on oppositesides of the positioning rod accommodating said guide rods to slidablysupport the brush holder with the ends thereof aligned with said coilfacings and to prevent rotation thereof, Contact brushes slidablydisposed Within the ends of the brush holder, contact blocks within theouter ends of the brush holder, coil springs disposed between thecontact blocks and the contact brushes resiliently biasing the contactblocks into engagement with the guide rods and the contact brushes intoengagement with the exposed facings, and jumper leads connecting thecorresponding contact blocks and contact brushes.

7. An adjustable rheostat, comprising a coil of resistance wire, asupport for the coil, a tubular brush holder, an axially movablepositioning rod mounted for axial susana@ movement through the coil,means securing the brush holder to the rod for selective positioning or"the holder, a pair of combination terminal and guide rods projectingthrough the coil on opposite sides of the positioning rod, said brushholder having guide rod openings accommodating said guide rods toslidably support the brush holder with the ends thereof aligned withselected portions of said coil and to prevent rotation thereof, andContact brushes slidably disposed within the ends of the brush holderand slidably engaging the adjacent portions ofthe coil.

8. The construction of an adjustable rheostat according to claim 7having contact blocks within the brush holder slidably engaging theguide rods, spring means disposed between the contact blocks and thecontact brushes rel siliently biasing the Contact blocks into engagementwith the guide rods and the Contact brushes into engagement with theexposed facings, and jumper leads connecting the corresponding Contactblocks and contact brushes.

Reorences Cited by the Examiner UNHED STATES PATENTS RICHARD M. WOOD,Primary Examiner.

1. IN AN ADJUSTABLE ELECTRICAL RESISTANCE DEVICE, A TUBULAR METALLIC CASING, A RESISTANCE WIRE COIL WITHIN THE CASING, A BONDING AND INSULATING MATERIAL SECURING THE COIL TO THE CASING, SAID COIL INCLUDING AT LEAST DIAMETRICALLY OPPOSED FACINGS FREE OF SAID MATERIAL, A BRUSH HOLDER SLIDABLY MOUNTED WITHIN THE CASING AND MOVABLE AXIALLY THEREIN, AND A PAIR OF CONTACTS RESILIENTLY SECURED IN THE BRUSH 